This invention relates to pneumatic servo boosters and, particularly to pneumatic servo boosters including a shell housing consisting of front and rear shells, a valve body incorporating therein a valve mechanism and sealingly extending through and being slidably supported by the rear shell, and a flexible diaphragm connected to the valve body and partitioning the interior of the shell housing into two front and rear chambers.
In a conventional vacuum servo booster, the two chambers are at a vacuum pressure in the non-actuated condition, and in actuating the servo booster by such as depressing a brake pedal, the valve mechanism firstly intercepts the communication between the two chambers and, secondly, introduces atmospheric air into the rear chamber thereby generating a differential pressure across the diaphragm. The diaphragm together with the valve body is displaced forwardly in generating an output force. The valve body sealingly extends through the rear shell and is slidably supported by the rear shell. A seal member is located on the inner surface of a rearwardly extending tubular portion of the rear shell, and the seal member is located between an annular flange formed on the rear end of the tubular portion and a resilient retaining ring which is fitted in the tubular portion. However, there are shortcomings in that the construction is complicated, the number of parts increases, the assembling operation is troublesome and the retaining force of the resilient retaining ring varies due to the manufacturing tolerance of the retaining ring and of the inner diameter of the tubular portion.